Non-destructive testing method of determining the depletion of a coating

ABSTRACT

It is disclosed a method of measuring the thickness and/or the depletion of Al and Cr of a γ/γ′ MCrAlY-coating after the use in a high temperature environment, the γ/γ MCrAlY-coating exhibiting a non-equilibrium γ/γ′-microstructure at a temperature lower than the temperature during operation. Before the coating conductivity by means of a multifrequency eddy current system is measured a heat treatment is applied to transform the non-equilibrium γ/γ′-microstructure to a equilibrium microstructure having a α-Cr phase. Subsequently the Al and/or Cr depletion of the coating from the coating conductivity and permeability is determined.

FIELD OF INVENTION

The invention relates to a non-destructive testing method of determining the depletion of a coating after use in a high temperature environment.

STATE OF THE ART

Components designed for the use in the area of high temperature, e.g. blades or vanes of a gas turbine, are usually coated with resistant coatings. The coating protects the base material against corrosion and oxidation due to the thermal effect of the hot environment and consists of an alloy mostly using the elements Al and Cr. Most turbine components are coated for the protection from oxidation and/or corrosion with, for example, a MCrAlY coating (overlay coating) and some are also coated with a thermal barrier coating (TBC) for thermal insulation. MCrAlY protective overlay coatings are widely known in the prior art. They are a family of high temperature coatings, wherein M is selected from one or a combination of iron, nickel and cobalt. As an example U.S. Pat. No. 3,528,861 or U.S. Pat. No. 4,585,481 are disclosing such kind of oxidation resistant coatings. U.S. Pat. No. 4,152,223 as well discloses such method of coating and the coating itself. Besides the γ/β-MCrAlY-coating, there is another class of overlay MCrAlY coatings which are based on a γ/γ′-gamma/gamma prime-structure. The advantages of γ/γ′-coatings over γ/β-coatings is that they have a negligible thermal expansion mismatch with Nickel-based superalloy of the underlying turbine article.

Among γ/γ′-coatings and γ/β-coatings, the field of γ/β-coatings have been an active area of research and a series of patents has been issued. E.g. a NiCrAlY coating is described in U.S. Pat. No. 3,754,903 and a CoCrAlY coating in U.S. Pat. No. 3,676,085. U.S. Pat. No. 4,346,137 discloses an improved high temperature fatigue resistance NiCoCrAlY coating. U.S. Pat. No. 4,419,416, U.S. Pat. No. 4,585,481, RE-32,121 and U.S. Pat. No. 4,743,514 describe MCrAlY coatings containing Si and Hf. U.S. Pat. No. 4,313,760 discloses a superalloy coating composition with good oxidation, corrosion and fatigue resistance.

In contrast to the γ/β-coatings, the γ/γ′-type of MCrAlY coatings, known e.g. from U.S. Pat. No. 4,973,445, are relatively new. The unique feature of this type of γ/γ′-coatings is that their thermal expansion mismatch is close to zero in combination with a high ductility, what make these coatings more resistant to thermal fatigue. However the limitations at elevated temperature are the lower aluminum content compared to γ/β-coatings.

For coatings the in-service degradation of the coating is one of the key factors for setting the time schedule for the periodic maintenance of gas turbines. In this context quantitative non-destructive characterization of high-temperature coating is an arising interest both to assess the quantity of the coating in the “as applied” condition and to obtain objective data on coating operational degradation that can be used to optimize blade refurbishment cycles.

Many services exposed turbine components are metallurgical investigated because of the main question of coating degradation. Non-destructive Testing (NDT) methods can provide essential information, such as residual coating lifetime, coating thickness distribution and any kind of delamination defects. NDT methods lower the need of time and cost consuming destructive metallurgical investigations. However due to the variability of the microstructure γ/γ′-coatings the applicability of NDT eddy current methods for estimating the effective thickness of service exposed coatings is not straight forward.

During an engine stop from the operating temperature down to below 600°, the γ/γ′ MCrAlY-coating exhibits a non-equilibrium γ/γ′-microstructure at room temperature due to the rapid cooling. Equilibrium phases, which are stable at low temperatures such as the α-Chromium phase, can not re-precipitate. The non-equilibrium microstructure of the coating results in a modified coating conductivity, which makes a reliable NDT coating assessment using the multi-frequency eddy current method impossible.

SUMMARY OF THE INVENTION

It is object of the present invention to find a heat treatment for service-exposed GT blading components with γ/γ′-coating to be able to subsequently apply a non-destructive testing method which is capable of determining the Al and/or Cr depletion within the γ/γ′-coating.

According to the invention a method of a heat treatment was found which enables the measurement of the depletion of Al and Cr of a γ/γ′ MCrAlY-coating. The inventive method comprising the steps of

-   (a) applying a defined annealing heat treatment to the γ/γ′     MCrAlY-coated component to transform the non-equilibrium high     temperature γ/γ′-microstructure into the equilibrium room     temperature microstructure with a α-Cr phase, -   (b) measuring the coating electrical conductivity and magnetic     permeability of the MCrAlY-coating by means of a multi-frequency     eddy current system and -   (c) determining the Al and/or Cr depletion of the coating from the     coating conductivity and permeability.

From the Al and/or Cr depletion of the coating the remaining life-time of the coating can be determined.

For a coating consisting of (wt.-%) 25% Cr, 5.5% Al, 1% Ta, 2.6% Si, 0.5% Y, Rest Ni and unavoidable impurities a transformation heat treatment, which guarantees the full restoration of the equilibrium condition of the coating, must be applied.

A heat treatment at temperatures above 1000° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10 K/min from the heat treatment temperature down to below 800° C. is required to guarantee the formation of the equilibrium microstructure. A long-term exposure at temperatures between 800°–870° C. for 16 to 24 hours can also be applied for the restoration of microstructure.

BRIEF DESCRIPTION OF DRAWINGS

This invention is illustrated in the accompanying drawing, in which

FIG. 1 shows a turbine blade,

FIG. 2 shows the coating structure after exposure with rapid cooling afterwards resulting in non-equilibrium microstructure,

FIG. 3 shows the coating structure after the new annealing temperature resulting in an equilibrium microstructure and

FIG. 4 illustrates a schematic drawing of a multifrequency eddy current system.

The drawings show only parts important for the invention.

DETAILED DESCRIPTION OF INVENTION

As an example FIG. 1 shows an article 1 such as blades or vanes of gas turbine engines, the gas turbine blade comprising a root portion 2, a platform 3 and a blade 4 and cooling holes 5. On the external surface 7 a MCrAlY-coating 6 is applied. The component can be made from a Nickel base superalloy known in the state of the art, e.g. from the document U.S. Pat. No. 5,888,451, U.S. Pat. No. 5,759,301 or from U.S. Pat. No. 4,643,782, which is known as “CMSX-4”.

The invention relates to a method for estimating the thickness and the depletion of protective elements such as Al and Cr of a high temperature MCrAlY-coating 6—like for example SV20 coating—after the use of this component in a high temperature environment.

The chemical composition (wt.-%) of the SV20-MCrAlY coating, which is known from DE-C1-37 40 478 or U.S. Pat. No. 4,973,445, is shown in Table 1:

TABLE 1 Coating Ni Cr Al Ta Si Y SV20 Balance 25 5.5 1 2.6 0.5

After the standard heat treatment (1120° C./2 h+870/20 h) the SV20 coating shows a microstructure consisting of a γ-Ni-matrix with the Al rich γ′ phase and Cr-rich α-Cr phase. The equilibrium volume fraction of the α-Cr phase in the initial state SV20 is approximately 20%. The γ phase is the Al reservoir phase for the formation of alumina to protect against high temperature oxidation above 900° C. The α-Cr phase is the Cr reservoir phase for the formation of Cr-Oxide to protect against low temperature oxidation and corrosion.

Unlike a MCrAlY with a γ/β-structure, which is stable on a wide temperature range, the microstructure of the non-depleted SV20 coating is strongly dependent on the temperature the component is subjected to during operation. Besides the microstructural changes in the non-depleted SV20 coating as a function of the operating temperature also the oxidation and outer depletion behaviour of the SV20 changes with temperature.

At temperatures between 750° and 900° C. the SV20 coating consists of the γ-Ni-matrix, the γ′ and the α-Cr phases. The SV20 coating does not show a significant microstructural change during cooling from temperatures below 850° C. down to Room temperature (RT). Mainly chromium-oxide forms at the outer surface of the SV20 coating which leads to a chromium depleted layer in the SV20 coating. The degradation level of the SV20 coating can be metallographically investigated by measuring the loss of α-Cr phase in the coating. The coating is considered exhausted (conservative approach) when a Cr content lower than 12–15 wt % is reached.

At temperatures above 900° C. the α-Cr phase starts to dissolve and with increasing temperature and time the fraction of the α-Cr phase decreases permanently until the α-Cr phase is completely dissolved. The cooling rates during an engine stop are generally too high for a re-precipitation of the α-Cr phase. This means that the microstructure of a SV20 coating, which was subjected to temperatures higher than 900° C., shows a lower α-Cr phase content compared to its equilibrium condition. The SV20 coating depletes at this temperature mainly from Al as the SV20 coating forms Al-oxides as the protective oxide scale. The degradation level of the SV20 coating above 900° C. can be metallographically investigated by measuring the thickness of the γ′ free layer. The coating is considered exhausted when an Al content lower than 3 wt % is reached.

At elevated temperatures of above 1000° C. a phase transformation according to α+γ′⇄β+γ takes place. The α-Cr phase is entirely dissolved and the γ and Al-rich β phase are in an equilibrium condition. During an engine stop the cooling rates are generally too high for a complete re-transformation to the equilibrium microstructure at RT. Such SV20 coatings, which were subjected to temperatures above 1000° C. in service, show a non-equilibrium microstructure at RT consisting of all four phases: α+γ′+β+γ.

In order to assess the influence of the presence, fraction and size of the α, γ′, β phases in the γ-matrix on the conductivity and permeability of the SV20 coating, tests were performed on the following two sets of samples:

Set1. 6 samples of “as applied” SV20/MarM247 (subjected to the standard heat treatment at 1120° C./2 h+870° C./20 h) were annealed for 60 hours at the following temperatures: 800° C., 850° C., 900° C., 950° C., 1000° C. and 1050° C. All samples were then quenched in air resulting in high cooling rates in order to maintain the non-equilibrium γ/γ′-microstructure at room temperature. FIG. 2 shows the structure of the coating after exposure to above mentioned heat treatments.

Set2. 6 samples of “as applied” SV20/MarM247 were treated as the previous set1 samples, and afterwards subjected to an additional heat treatment at 800° C. for 20 h. The additional heat treatment allowed the SV20 coatings with non-equilibrium γ/γ′-microstructure to re-transform to the equilibrium microstructure with a α-Cr phase. FIG. 3 shows the structure of the coating after the new annealing temperature resulting in an equilibrium microstructure.

Both sets of SV20 samples were measured before and after the long-term exposure at several temperatures using a multifrequency eddy current system as described as an example in Antonelli, G., Crisafulli, P., Tirone, G., 2001, “Qualification of a Frequency Scanning Eddy Current Equipment for Non-destructive Characterization of New and Serviced High-Temperature Coatings”, ASME paper No. 2001-GT-0419 and Antonelli, G., 2002, “Non-Destructive Condition Assessment of MCrAlY Coatings”, presented at Turbine Forum on Advanced Coatings for High Temperatures, Nice, France. This system was developed for non-destructive condition assessment of new and service exposed high-temperature coatings applied on the hot gas-path components of land-based gas turbines.

A schematic view of the system is given in FIG. 4. It is a portable system consisting of following modules:

-   Probe with combined or separated transmit-receiver coils, -   Eddy current frequency scanning range from 100 kHz to MHz, -   Feeding and conditioning unit with high sensitivity and high     stability, -   Computer based data analysis unit and -   Display unit

The system fully exploits the capabilities of multiple-frequency eddy current techniques in the characterisation of multiple-layer materials, based on the evaluation of even quite small differences in the electrical conductivity values of the different layers.

The influence of the non-equilibrium microstructure from the coatings aged above 900° C. on the eddy current data is significant. Those coatings seem to be heavily depleted and degraded due their non-equilibrium structure. With increasing ageing temperature the fraction of the α-Cr phase decreases permanently which results in a lower normalised impedance curve obtained by the frequency scanning eddy curry system. At temp. >950° C. the β phase occurs which then results again in an increase of the normalised impedance curve. The normalised impedance curve for the 950° C.-exposed sample is the lowest. After the additional heat treatment at 800° C./20 h, resulting in a re-transformation of the non-equilibrium to the equilibrium microstructures, the normalised impedance curves return to be similar to ones obtained for the as-applied condition.

The above qualitative analysis of the impedance curves is confirmed by model based analysis of the impedance curves giving estimates for coating and substrate conductivity. There is a strong dependence of the coating conductivity on the exposure temperature as estimated by the frequency scanning eddy current system, especially in the range from 800° C. to 950° C., where electrical conductivity correlates well with Cr dissolution. This effect poses a problem in the interpretation of eddy current data from service exposed blades. The coating conductivity depends on two effects, of which only the first is related with real coating consumption:

-   Cr-depletion as a consequence of coating oxidation (depletion     effect); -   α-Cr dissolution as a consequence of non-equilibrium microstructures     obtained at different operating temperatures (microstructural     effect).

Therefore an estimation of the effective remaining coating thickness by the eddy current technique is only possible with a certain accuracy if the SV20 MCrAlY coating is brought back to its equilibrium microstructure by applying an annealing treatment whereas estimation of the exposure temperature can be performed if the degradation is known (e.g. no degradation for short exposure time). From the Al and/or Cr depletion of the coating the remaining life-time of the coating can be determined.

At temperatures higher than 900° C., where the degradation mechanism is Al depletion, the capability of the eddy current technique to grade coating depletion was not investigated. However problems are expected due to:

-   non monotonic conductivity dependence on the exposure temperature; -   the low Al content in the coating and the low conductivity variation     expected from its complete depletion (form 5.5 wt.-% in the new     coating to below 3 wt.-% in the exhausted coating).

The observation that the additional heat treatment aiming at re-transforming the coating to its equilibrium microstructure cancels the microstructural effect on coating conductivity can be exploited by applying an additional heat treatment to the samples before testing them with the eddy current technique.

For the material tested such heat treatment according to the present invention can be at temperatures above 1000° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10K/min from the heat treatment temperature down to below 800° C. in order to guarantee the formation of the equilibrium microstructure. A long-term exposure at temperatures between 800°–870° C. for 16 to 24 hours can be applied for the restoration of microstructure.

While our invention has been described by an example, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of our invention is to be limited only by the attached claims.

REFERENCE LIST

-   1 Article -   2 Root portion -   3 Platform -   4 Blade -   5 Cooling holes -   6 Coating -   7 External surface of article 1 

1. A method of determining the depletion of Al and Cr of a γ/γ′ MCrAlY-coating of a component after use in a high temperature environment, the method comprising: (a) using a component having a γ/γ′ MCrAlY-coating in a high-temperature environment in which the γ/γ′ MCrAlY-coating exhibits an equilibrium γ/γ′-microstructure, (b) cooling the component to a temperature lower than the operation temperature such that the γ/γ′ MCrAlY-coating exhibits a non-equilibrium γ/γ′-microstructure at room temperature. (c) applying a defined annealing heat treatment to the γ/γ′ MCrAlY-coated component to transform the non-equilibrium high temperature γ/γ′-microstructure into the equilibrium room temperature microstructure with a α-Cr phase, (d) measuring qualitatively impedance curves or the electrical conductivity and magnetic permeability of the MCrAlY-coating by means of a multi-frequency eddy current system, and (e) determining the Al and/or Cr depletion of the coating from the measured impedance curves or coating conductivity and permeability.
 2. The method according to claim 1, wherein from the Al and/or Cr depletion of the coating the remaining life-time of the coating is determined.
 3. The method according to claim 1, wherein the coating consists of (wt.-%) 25% Cr, 5.5% Al, 1% Ta, 2.6% Si, 0.5% Y, rest Ni and unavoidable impurities.
 4. The method according to claim 1, wherein a transformation heat treatment at a temperature of 800°–870° C. for 16 to 24 hours is applied.
 5. The method according to claim 1, wherein after the operation the coating is heat treated with a temperature above 1000° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10 K/mm from the heat treatment temperature down to below 800° C. to transform the microstructure.
 6. The method according to claim 2, wherein the coating consists of (wt.-%) 25% Cr, 5.5% Al, 1% Ta, 2.6% Si, 0.5% Y, rest Ni and unavoidable impurities.
 7. The method according to claim 2, wherein a transformation heat treatment at a temperature of 800°–870° C. for 16 to 24 hours is applied.
 8. The method according to claim 3, wherein a transformation heat treatment at a temperature of 800°–870° C. for 16 to 24 hours is applied.
 9. The method according to claim 2, wherein after the operation the coating is heat treated with a temperature above 1000° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10 K/mm from the heat treatment temperature down to below 800° C. to transform the microstructure.
 10. The method according to claim 3, wherein after the operation the coating is heat treated with a temperature above 100° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10 K/mm from the heat treatment temperature down to below 800° C. to transform the microstructure.
 11. The method according to claim 4, wherein after the operation the coating is heat treated with a temperature above 1000° C. for at least 2 hours with a subsequent controlled cooling rate of 2–10 K/mm from the heat treatment temperature down to below 800° C. to transform the microstructure.
 12. The method according to claim 1, wherein the impedance curves of the coating are measured.
 13. The method according to claim 1, wherein the electrical conductivity and magnetic permeability of the coating are measured. 